1. Field of the Invention
The present invention relates to an electronic component, and more specifically to an electronic component provided with an external terminal including a lead base material constituted of a predetermined metal material, and a metal thin film coating a surface of the lead base material and including at least a first layer constituted of a material substantially Pb-free and predominantly composed of tin, and to a method of manufacturing such electronic component.
2. Description of the Related Art
When implementing an electronic component such as an integrated circuit (hereinafter abbreviated as “IC”), a transistor, a resistance, a condenser and so forth on a circuit board or the like, an external terminal of the electronic component is electrically connected to a conductive electrode of the circuit board, via a low-melting solder. In this process, the external terminal of the electronic component has to have sufficient solder wettability, so as to secure desired connection reliability between the electronic component and the circuit board. For this purpose, a surface of the lead base material of the external terminal is provided with a metal thin film constituted of Sn or an Sn-based alloy mainly composed of Sn, formed in advance by a surface finishing process such as electrolytic plating.
Referring to the low-melting solder, an Sn—Pb alloy mainly containing tin (hereinafter designated by “Sn”) and an additive of lead (hereinafter designated by “Pb”) has been popularly used. Here, Sn is the main ingredient of the alloy and serves as an adhesive. Also Pb constitutes, together with Sn, a group of metals that can form a low-melting alloy, and serves to lower a melting point of the alloy and to enhance an adhesion strength thereof. Accordingly, an Sn—Pb alloy has been widely utilized in implementing an electronic component on a circuit board, because of the advantage that a melting point can be easily controlled by adjusting a ratio of those two ingredients, and that these metals are inexpensive.
Referring also to the metal thin film to be formed on a surface of a lead base material of an external terminal included in an electronic component, the metal thin film formed by plating an Sn—Pb alloy has been popularly employed. It is because of the excellent wettability with a low-melting Sn—Pb solder as well as of the inexpensive cost that an Sn—Pb alloy has been preferably employed for plating to form a metal thin film on a surface of a lead base material for an external terminal.
However, the Pb ingredient in the Sn—Pb alloy is harmful against a human body, and besides discarding a used electronic device incurs environmental pollution. Accordingly, Pb is not desirable from an environmental viewpoint. Lately, therefore, a so-called Sn-based and substantially Pb-free alloy, which excludes Pb as an ingredient for constituting a low-melting solder, has come to be popularly used when implementing an electronic component on a circuit board. In correspondence with such trend, on the part of the lead base material of the electronic component also, a Sn-based and substantially Pb (lead) free alloy has come to be widely used for plating a surface thereof, to form a metal thin film.
Here, “substantially Pb-free” means a concentration of Pb of less than 1 wt %, preferably 0.1 wt %. On the other hand, “Pb contained material” contains Pb of 5 to 10 wt %.
When plating a Sn-based and substantially Pb-free alloy on a surface of a lead base material to form a metal thin film, the key issue is maintaining a desired low-melting solder wettabillty and securing desired connection reliability, whichever metal may be adopted as an additive to Sn.
However in case where a Sn-based and substantially Pb-free alloy is plated on a surface of a lead base material to form a metal thin film, a fine metal whisker are prone to appear on a surface of an external terminal under a circumstance of practical use of the electronic component, unlike a case of forming a metal thin film by an Sn—Pb alloy. Such whisker may cause a short circuit between the external terminals, and the tendency becomes higher especially in an electronic component such as an IC in which a multitude of external terminals are led out at fine intervals from a peripheral portion of the package body. Accordingly, restraining emergence of a whisker constitutes a critical issue when forming an outer coating of a metal thin film by plating Sn or a Sn-based and substantially Pb-free alloy on a surface of a lead base material of external terminals disposed in an electronic component.
Referring to this issue, models of whisker emergence mechanism, as well as methods of restraining whisker emergence for the respective models thus far proposed will be described below.
For example, JP-A Laid Open No.2002-246208 refers to a whisker that emerges on an external terminal of a variable resistor. More specifically, the external terminal is provided with a first plated layer consisting of copper (hereinafter designated by “Cu”) formed on a surface thereof, and a second plated layer consisting of Sn of 1 to 4 μm in particle diameter, formed on the first plated layer. The cited document states that a whisker emerges after long hours of use of such variable resistor, thereby causing the variable resistor to render an unstable output. FIG. 10 is a schematic cross-sectional view showing a state that a whisker has merged on an external terminal of the variable resistor. According to the JP-A Laid Open No.2002-246208, the external terminal 113 is provided with a metal plate portion 109, the first plated layer 110 consisting of Cu formed thereon, and also the second plated layer 111 consisting of Sn formed on the first plated layer 110. Because of such structure, an oxide layer emerges on a surface of the second plated layer 111 after using the variable resistor for a long time, thereby causing expansion of each particle constituting the second plated layer 111, resulting in emergence of whisker 114 on the external terminal 113, as shown in FIG. 10. And in order to prevent the whisker from emerging in this way on the second plated layer, the JP-A Laid Open No.2002-246208 proposes fusing the plated particles of the second plated layer to transform the particles into a uniform layer, after forming the first plated layer consisting of Cu and the second plated layer consisting of Sn on the external terminal surface. The document states that this process can prevent expansion in volume of the plated particles due to formation of an oxide layer despite a long term use of the variable resistor, and can resultantly restrain the whisker emergence on the second plated layer consisting of Sn formed on the external terminal surface.
Also, JP-A Laid Open No.2001-110666 discloses providing an Sn-plated layer consisting of fine particles not larger than 1 μm in average crystal particle diameter, on an outermost layer of an external electrode consisting of a plurality of layers, formed on a base material surface of an electronic component. More specifically, the external electrode includes a nickel (hereinafter designated by “Ni”) plated layer formed on a thick film electrode located in contact with both end portions of a ceramic base material, and an Sn-plated layer being formed on the Ni-plated layer and constituting an outermost layer of the external electrode. The Sn-plated layer is formed by electrolytic plating of Sn, such that an average crystal particle diameter becomes 1 μm or less. Such fine structure can restrain whisker emergence on the Sn-plated layer, even under a circumstance of a temperature cycle alternately repeating a high temperature and a low temperature.
Also, Ryusuke Kawanaka et al., “Role of Lead in growth suppression and growth mechanism of Tin-proper whisker”, Japanese Association for Crystal Growth, Vol. 10, No.2, pp. 148-156, Dec. 15, 1983 reports different emergence mechanisms of a whisker.
FIG. 11 is a schematic drawing for explaining a whisker emergence mechanism stated in Ryusuke Kawanaka et al., “Role of Lead in growth suppression and growth mechanism of Tin-proper whisker”. Japanese Crystal Growth Society. Vol. 10, No.2, pp. 148-156, Dec. 15, 1983. According to this literature, an oxide layer grows with the lapse of time on a surface of an Sn or an Sn-alloy plated layer formed on a metal surface. Since such oxide layer often has an uneven thickness, an internal stress produced inside the plated layer concentrates in a defective portion of the oxide layer where the layer is thin, and squeezes out inner atoms, thereby causing a whisker to grow.
However, the method of fusing the plated particles in the Sn-plated layer according to the JP-A Laid Open No.2002-246208 includes, for example, placing internal and external terminals in a first furnace provided with an FIR heater for approx. 30 seconds for preheating up to 220 degree centigrade, and passing them through a second heating furnace provided with a burner in approx. one second, for heating up to approx. 900 degree centigrade. Treating at such a high temperature a semiconductor component provided with a resin material, which is less resistant against heat than a metal, at a position close to a terminal incurs heat degeneration of the resin component (melting, carbonization, degradation, oxidation and so on), deterioration of materials of each configuration, mechanical damage by vaporizing expansion of water in the package, irregular growth of the interfacial metal alloy layer to cause degradation in reliability of performance of the component.
Also, the JP-A Laid Open No.2001-110666 proposes forming an Sn-plated layer consisting of fine crystal particles of not larger than 1 μm in average diameter, to thereby restrain whisker emergence under a temperature cycle. However, some data have shown a contradictory result that a whisker is more prone to appear when a crystal particle diameter of the Sn-plated layer is smaller (For example, JP-A Laid Open No.1990-170996).
According to studies on the whisker emergence mechanism so far achieved, including the foregoing JP-A Laid Open No.2002-246208, JP-A Laid Open No.2001-110666. JP-A Laid Open No.1990-170996, and Ryusuke Kawanaka et al., “Role of Lead in growth suppression and growth mechanism of Tin-proper whisker”, Japanese Crystal Growth Society, Vol. 10, No.2, pp. 148-156, Dec. 15, 1983, it is generally considered that formation of an interface reaction layer and a surface oxide layer, internal compression stress in a plated layer, recrystallization, defect displacement, unmatched thermal expansion coefficient between a base material and a plated layer, a compression stress generated by a mechanical processing and so forth are promoting factors for the whisker growth. FIG. 10 shows one of the reported models, where upon formation of a thin oxide layer on an Sn-plated surface, an internal stress produced within the Sn-plated layer concentrates in a defective portion of the oxide layer and easily squeezes out inner atoms, thereby permitting a whisker to emerge. Various approaches have been made as stated in the JP-A Laid Open No.2002-246208, JP-A Laid Open No.2001-110666, JP-A Laid Open No.1990-170996, and Ryusuke Kawanaka et al., “Role of Lead in growth suppression and growth mechanism of Tin-proper whisker”, Japanese Crystal Growth Society, Vol. 10, No.2, pp. 148-156, Dec. 15, 1983, and yet exact details of the whisker emergence mechanism have not been clarified.
Nevertheless, from the viewpoint of environmental protection, it is necessary to promote the use of a substantially Pb-free material for plating an outer surface of an external terminal of an electronic component. For this purpose, it is essential to restrain the whisker emergence under a circumstance of practical use, effectively and at a low cost.